1. Field of the Invention
This invention relates to processor and memory modules, and more particularly for modules and production methods to reduce the cost of CPU/SRAM modules.
2. Description of the Related Art
Memory modules containing dynamic random-access memories (DRAM's) are shipped with most personal computers (PC's) today, allowing for later expansion with additional modules. Occasionally the central processing unit (CPU) or microprocessor is mounted on a small daughter or processor card or module that is inserted into a socket in the PC's motherboard. Using processor modules allow the CPU to be upgraded by replacing the processor card.
These processor modules often include a small cache memory, such as a first or second-level cache. A cache memory reduces memory latency by providing a small storage area for the most-recently-used data and instructions. Locality of instructions and data predicts that the most-recently-used data and instructions are likely to be used again. Since the cache memory is faster than the main DRAM memory, data in the cache is fetched more rapidly than data in the main DRAM memory.
Static RAM chips are used for cache memories. The access delay for SRAM chips is significantly smaller than for DRAM chips, as SRAM chip access delays are currently under 10 nanoseconds, while DRAM access delays are 50 or 60 nanoseconds. SRAM chips are simpler to interface, as they do not require refreshing, as do DRAM chips.
Like memory modules, processor modules are made in many different sizes and capacities, with different numbers of interface pins. The "pins" were originally pins extending from the module's edge, but now most modules are leadless, having metal contact pads or leads. The modules are small in size, being about 3 to 5 inches long and one or two inches high. The modules contain a small printed-circuit board substrate, typically a multi-layer board with alternating laminated layers of fiberglass insulation and foil or metal interconnect layers. Surface mounted components are soldered onto one or both surfaces of the substrate. Static-RAM integrated circuits (IC's) or chips are commonly packaged in inexpensive surface-mount packages such as small-outline J-leaded (SOJ) packages, PLCC's, or small-outline (SO) packages.
The SRAM chips draw significant amounts of current when address signals change and when OE (output enable) is asserted during accessing of the memory. Asserting OE or other control signals, and changing address inputs cause current spikes, which could upset the power and ground potentials on the processor module. Thus, capacitors are often mounted on the substrate near each SRAM chip. The capacitors are connected between power and ground as close as possible to the power and ground pins of each SRAM package. For wide SRAM chips with multiple power and ground pins, several capacitors may be used for each chip.